Help with trouble shooting a failure

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Sep 12, 2006
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#1
Hey Fellas,
If I were to post up some pics of a top end failure, would some of you guru's be willing to help me out with what caused the failure? I really don't want to repeat this problem if I can avoid it.
Thanks in advance, Griffbones
 
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#4
2007 KX100 engine, pipe, electrics and carb in a 2005 KX85 bike. 13/50 gearing with the short wheels of the KX85, needless to say it is geared pretty deep. We ride tight woods and my son is NOT very agressive thus the deep gears and a flywheel weight.
Last week I mixed up a fresh batch of 91 octane pump fuel mixed with Amsoil Dominator at 40:1. We ride for about an hour or so then head down a pretty long straight away. My son always rolls the throttle and then backs off and the rolls on then backs off on real long straights becuase I have told him not to leave it pinned on a real long (1/8 - 1/4 mile) straight. Well all at once the bike dies on him, but restarts but doesn't want to idle. We get home and the bike starts right up and idles fine. A compression test reveals 165-170 PSI! But the top end sounds noisey, so I pull it apart and find this:


Now this bike is jetted 100% stock, so I would have thought that it would have been a little rich. What few times he has ridden the bike the plug porcelin always looked tan with a bronze color to the base of the threads and on the electrodes.

The bike still seemed to run very strong and as stated had 165+ for compression but the top end had a rattle, so that is why I tore it down and found this problem.

Yesterday per the service manual, I smoothed up the cylinder with 440 grit wet dry sand paper, and then cleaned the cylinder with brake cleaner, then soapy water, then rinsed and then dried it off with compressed air. I then installed a new Wiseco piston and top bearing. Started her up and she sounded great. I let it cool down completely, then repeated this warm up/ cool down cycle, and then took it for an easy five minute ride. Then I let it cool down again and then started another easy ride but only made it about 10 minutes top, and then she seized up again. This was with no hard, only moderate throttle action. This time here is what I found, and the inside of the cylinder looked like it was full of gray lapping compound, unlike the first tear down which looked clean.


And here is what the plug looked like at each tear down, dark gray and almost powder coated, instead of the nice tan color that I had been accustomed to.


What the heck is causing this? And sorry for the long winded post, just trying to accurately describe the events. BTW this engine probably only had about 8 hours on it before the first failure. No jetting was changed so I don't think it should have been lean. And as I stated this was fresh gas with synthetic oil at 40:1.
Thanks a bunch guys, Griffbones
 
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#5
did you let the bike warm up before he went out and flogged it ? it looks to me like the piston expanded and stuck you can kinda see it was a 4 corner siezure. also if you installed a wiseco piston, heres what you have to do to break it in correctly this is what a wiseco rep told me . start it up and let it idle till it gets up to temp then shut it down and repeat the process 3 more times this is because the forged piston has a high silicone content in it and a fast expasion rate and i guess going through a few heat cycles must cure the piston from expanding to fast . oh and the only way you can find this out is if you call wiseco its not the instructions that come with the piston, which is kinda weird
 
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#6
Anyone notice the weird mark in the first and third pic that looks like an upside down boot?
 
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#7
I did two warm up cycles and then a real easy five minute ride, then after another cool down I rode it for about ten minutes max at only partial throttle. And what caused the first failure? This thing had a tan plug when it was 50 degrees outside, but it failed on a 75 degree day, should have been even richer, no?

That imprint is where the piston appears to have drug on the intake port and rolled the metal up.

Could a CDI box go bad and throw the timing wacko and cause it to heat?

The first failure bothers me more than the second because the plug had been running so good and I just don't see the reason for that failure. I maybe should of had the cylinder re-plated before going another round, but I didn't think it looked that bad. After the second failure there was plenty of oil film in the cylinder but it was full of gray paste!
 
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#8
do you have a bad crank seal , it could be suckng air and running lean ? just a guess but i'd pull the flywheel and just check . also did you check the crank somtimes the crank bearing will feel ok but if you check it close you'll see that the cage is broken and the needles are all loose on the pin, again just a guess but i've run into it before .
 
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#9
I had a CDI box go bad on my Yamaha which made the timing run way advanced at idle. But... I would expect some crown damage if timing were the cause of failure. 40:1 oil ratio is pretty lean for a 100 but I don't think that's the problem. Did you happen to measure the bore clearance before you put the new piston in?
 
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#10
76GMC1500 said:
I had a CDI box go bad on my Yamaha which made the timing run way advanced at idle. But... I would expect some crown damage if timing were the cause of failure. 40:1 oil ratio is pretty lean for a 100 but I don't think that's the problem. Did you happen to measure the bore clearance before you put the new piston in?
Most of the local guys are running 40:1 & 50:1 on the 100's and the 85's when using the Dominator oil. I am always confused by this because some people say to run the bike manufacturers specs, and others like Eric Gorr say to run what the oil manufacturer recommends for their product because different oils are different.

I didn't have the equipment (bore gauge) to measure the cylinder, but I did check the ring gap. What made this thing loose the first bone stock top end? What the heck changed from the previous times we had the bike out? This was an ultra low hours engine (brand new 2007 KX100 engine). I changed nothing as far as set-up and I always do maintenance ie. air filters, tranny oil, fresh premix, a compression test after every couple of rides, and many of my son's rides are short, like only an hour or two tops.
 
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#11
i am gonna have to disagree 40:1 is fine i ride with alot of good riders (chris carter, rodney smith, kevin cullen, ray abrams) and they all mix the gas in their 2 strokes at 50:1 or higher. They recommended 50:1 for my old 85 and thats pretty close to a 100 so the mixture is not the problem
 
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#12
To me it looks like this thing got real lean real quick for an un-known to me reason, or it got real hot for some reason. The piston domes looked fine, in fact the original piston looked exactly like the photos Eric Gorr used to have up on his site, the ones where he says the transfer ports are nice and even and an example of good jetting!

The bike was not loosing any coolant either, so I don't think it got hot from that stand point.

The bike didn't sound lean either, except on the second top end, just a minute or so before it blew. I thought at the time it sounded higher pitched and not as deep sounding in the exhaust note. On the first top end I couldn't say, as I was following my son and couldn't hear his engine very well over my four stroke.

Man I just don't want to completely rebuild this thing top and bottom end just to have the same thing happen again, the wife is starting to get P-Oed at all the money I am spending on this bike. Especailly since our old 1983 CR60 has been just about bullet proof, and this new bike (bought last July as a left over 2005) lost the first KX85 engine to a crank failure almost imeadiately. That is how I happen to buy and put in this new 2007 100 engine.
 
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#13
I would take it to the dealer. Explain what has happened, bring the used up pistons in too for reference if you want. Have them do a leak down test to check for airleaks. Although It doesn't look like a timing issue, ie, holes burnt in the piston, burned out rod bearings, it wouldn't hurt to have them take a timing light to it just to double check. I would also inspect your cooling system. I am not familiar with it on that bike but the manual will tell you what to do. It also can't hurt to pull the carb and give it a good once over, you never can be too sure. good luck
 
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#14
I would think an overheating would have more of the scuffing towards the top of the piston, in the area around the rings. It really does look like lack of lubrication to me. How does the underside of the crown look, any discoloration? Here is a shot of the underside of a crown that has been running hot. You can actually see the location of the spark plug in the center of the crown and you can see that the exhaust side of the piston was too hot.